The present invention relates to an optical active matrix display of a type that uses an optical scanner having a rotating polygon mirror. The present invention is particularly suitable for large-area liquid crystal displays and can be used in various kinds of audio-visual (AV) equipment and office automatic (OA) equipment.
Having the ability to produce high-quality pictures, a liquid crystal display of active matrix is becoming part of the mainstream of liquid crystal displays. However, thin-film transistors (TFT) of which the active matrix is formed are fabricated by a complicated sequence of process steps and defects are prone to occur mainly in crossing areas of the metallization. The yield of production is lowered by the defects, and it is difficult to reduce the manufacturing cost thereof.
In accordance with the demand for large capacity display equipment, recently it is necessary to increase the number of pixels from 400.times.600 to 1,000.times.1,000 or more and, at the same time, the display screen is required to increase the size from the current size 10 inches (254 mm diagonal) to a bigger size 20 inches (508 mm diagonal). In liquid crystal display device of an active matrix type, particularly using of thin-film transistors (TFT), the length of metalized lines is that the resistance of metalized line increase accordingly, as a result, there is a problem of delayed signal waveforms due to the resistance of metalized lines and stray capacity thereof. Alternately, a liquid crystal display device of a simple matrix type is-not free from a problem; in proportion to increment of the scanning line accompanying to a screen enlarged, a difficulty insuring the voltage ratio (duty ratio) between pixels selected and non-selected increases, and the difficulty results in deteriorated display characteristics.
The following apparatus is provided to solve these problems, that is an optical active matrix display as shown in FIGS. 6 and 7 (Japanese Patent Unexamined Publication No. Hei 1-173016, etc.). As shown, the display comprises a glass substrate 101 having a plurality of parallel lightguide paths 102 formed in the surface thereof; a plurality of photoconductive material elements 103 are provided over the lightguide paths 102 in contact with the light channels which are deficient of the cladding so that part of the light passing along the lightguide paths 102 will be emitted toward the photoconductive material elements 103 overlying thereon. The individual photoconductive material elements 103 perform on-off control to switch back and forth between strips 104 and strips 105 in the form of a thin metal film, thereby working as optical switch elements 106. One group of strips 105 are connected to the pixel electrodes 107 which form pixels in liquid crystal display and an image electric signal for liquid crystal display is applied to the other group of strips 104, whereby said image electric signal is applied to the pixel electrodes 107 via the optical switch elements 106. In the case shown in FIG. 7, the strips 104 are formed in the horizontal direction whereas the lightguide paths 102 are formed in the vertical direction. When light is applied on the lightguide paths 102, the overlying optical switch elements 106 will turn on and the electric signal carried on the metal strips 104 is applied to the pixel electrodes 107, thereby controlling the molecular alignment of the liquid crystal. Suppose here that light is incident on the lightguide paths 102 in the order from the left end of FIG. 7. If light is first applied into the lightguide path 102 at the left end, all of the optical switch elements 106 provided over the particular lightguide path 102 will turn on but other optical switch elements 106 provided over the other rows of lightguide path 102 remain off state. Then, a picture signal (electric signal corresponding to pixel information) is applied to the strips 104, whereupon the same electric signal is applied to the pixel electrodes on the left row in FIG. 7, whereby the molecular alignment of the internal liquid crystal is controlled. The same applies to the second-to-the-right row of pixel elements 107. By repeating the procedure just described above, the picture signal is applied successively to all pixel electrodes 107 and the molecular alignment of the liquid crystal is properly controlled to display a desired picture.
Optical active matrix displays of the type described above require that light be applied successively into the individual lightguide paths. To meet this requirement, it has been proposed that light-emitting devices (e.g., LEDs, LDs and EL devices) be formed in association with the individual lightguide paths.
While there are several methods for scanning light, one way is to scan laser light with an optical system including a rotating polygon mirror, etc., as is typically effected in laser printers. The scanning system of a laser printer is shown in FIG. 12, in which a laser beam 140 reduced to a spot size of about 100 .mu.m is focused by convergence lens 150 for scanning over the surface of a photoreceptor drum 110 by a rotating polygon mirror 122.
A problem with the above-described system in which a light-emitting device is provided for each lightguide path is that at least several hundred and, usually, more light-emitting devices are necessary to fabricate liquid crystal display of ordinary size. This causes difficulty in assuring uniformity in the intensity of emission from the individual devices and it is not only cumbersome to mount such a great number of devices but there also occurs a problem in that the scanning light source composed of these devices is considerably too expensive. Furthermore, the necessity of controlling the light emission from the individual devices in time sequence cases inevitably to require a complicated control circuit.
The light scanning system which is conventionally used with laser printers has several features; to mention a few, laser light is subjected to intensity modulation in accordance with the desired information to be displayed, and the f.theta. lens which is used as a focusing lens to concentrate light on the scan surface of a photoreceptor drum need not be closely controlled in terms of the angle of principal rays that are receipt on the scan surface of the photoreceptor drum. Therefore, as will become apparent from the detailed description disclosed later, a display of uniform quality cannot be presented by merely combining the known light source and optical system with the optical active matrix display of the type described above.